Control of weight during evaporation of samples

ABSTRACT

A method of controlling the evaporation of liquid in samples in an evaporating centrifuge, by monitoring the centrifugal force exerted on a sample holder containing a liquid sample having solid material dissolved or otherwise mixed therein. The centrifugal force is determined using a load cell, a strain gauge or, where relative movement between sample holder and rotor is permitted albeit with resilient restraint, the centrifugal force signal may be generated by a position sensing transducer. The speed of rotation is sensed by a further transducer and both force and speed signals are conveyed to a computer programmed to generate a process control signal for controlling the evaporation process therefrom. A preferred method of control involves determining the rate of change of weight with time and terminating the evaporation process when the rate of change drops to zero. Evaporation is assisted by heating the samples and the process control signals determine not only the speed of rotation, but also the heating of the samples. A weight signal can be computed from the force signal by reference to the speed signal which is proportional to the centrifugal force acting on the sample holder and therefore the sample.

FIELD OF INVENTION

This invention relates to a method of and apparatus for controlling theweight of samples dissolved or suspended in a liquid while they areevaporating in a vacuum. It is particularly applicable to samples incentrifugal evaporators.

BACKGROUND TO THE INVENTION

Samples to be evaporated in centrifugal evaporators are usually held inglass or plastic tubes or, sometimes, in a large number of small wellsin plastic blocks. The sample holders are mounted upon a rotatingassembly and spun at relatively high speed so that a considerablecentrifugal force is applied to them in an outward direction, whichforces the liquid to the lower part of the sample tubes and prevents anyfrothing or spitting of the liquid out of the sample tubes when a vacuumis applied. The spinning samples are held in a vacuum-tight chamber(referred to herein as a “chamber”) which is connected to a vacuumpumping device.

Centrifugal evaporators of this type are well known and many types areavailable commercially. One problem from which such evaporators suffer,is that it is very difficult to obtain a desired continuous read-out ofthe weight of the sample in the holders as the liquid is beingevaporated, since the holders are being spun at a high speed, typicallyat about 1400 r.p.m. The possibility has been considered of continuouslyweighing the whole evaporator during spinning. However, this involvesmeasuring a total weight of the order of 50 kg to an accuracy of about 1gm, which is a very demanding task.

Another problem arises when evaporation needs to take placesimultaneously for different solvents, or solvent mixtures of differingcompositions, in which the samples are dissolved or suspended. In thissituation those samples which are held in the more volatile solvents ormixtures will evaporate faster than the ones held in the less volatilesolvents, and this can lead to an excessive imbalance in the rotatingassembly, and consequent unwanted vibrations. This would also mitigateagainst the possibility of weighing the whole evaporator.

In most centrifugal evaporator machines such unwanted vibrations arearranged to trip an out-of-balance sensor to thereby stop the machine,but in machines without a sensor the vibrations can cause damage to themachine and even to the samples. Sometimes the vibration problem can beovercome by careful loading of the evaporator, or by stopping theprocess from time to time and rebalancing the load by adding liquid toempty samples or by rearranging the samples in the rotating assembly.Both these methods are tedious and time consuming.

It is an object of the present invention to enable the weight of asample in a centrifugal evaporator to be continuously and accuratelymeasured during evaporation.

It is another object of the invention to enable the operation of acentrifugal evaporator to continue despite a considerable imbalance offorces.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, a method ofevaporating a liquid sample contained in a sample holder which ismounted within a chamber and rotated by a rotor therein during theevaporation so that centrifugal force is exerted on the contents of thesample holder during the process whilst a pressure below atmospheric ismaintained in the chamber in manner known per se, so as to leave as aresidue any solid material dissolved or otherwise mixed in the liquidforming the sample, characterised by: mounting a transducer to monitorthe force acting on the sample holder relative to the rotor whenrotating at a given speed and obtaining a force signal therefrom,supplying the force signal to a computer means, programming the computermeans to compute a value equivalent to the centrifugal force exerted onthe sample holder due to rotation of the rotor at said given speed,further programming the computer means to compute a weight value fromthe force signal using the computed centrifugal force, and furtherprogramming the computer means to generate a control signal forcontrolling the evaporation process in dependence on the computed weightvalue.

In some circumstances the rotor may be rotating at constant speed, sothat the weight value can be computed for that particular speed.

Alternatively, however, the method may further comprise the steps ofmounting a second transducer to monitor the speed of rotation of therotor, obtaining a speed signal therefrom, and supplying the speedsignal to the computing means for computing said weight value.

Preferably the computing means is adapted to rotate with the rotor.

Preferably the computing means is programmed to convert the output ofthe sensor into a form suitable for transmission to an externalreceiver.

Preferably the computing means converts the transducer signals intodigital signals by which a carrier signal is modulated to effect thesaid transmission.

In general the transducer signals are produced continuously and theweight and centrifugal force factor values are continuously computedtherefrom.

Conveniently the computing means has stored therein a value equivalentto the weight of the sample holder, and is further programmed to computea value equivalent to the weight of the contents of the holder bydeducting from the computed weight value a value equivalent to the knownweight of the sample holder.

Preferably the computer means computes the rate of change of thecomputed weight value.

Preferably the method includes the step of heating the sample duringrotation in the chamber to increase the rate of evaporation.

Preferably the method includes the step of controlling the supply ofheat to the sample in dependence on the computed weight value,preferably in dependence on the computed rate of change of weight value.

In general, the supply of heat will be reduced as the rate of change ofweight with time starts to decline, and the evaporation process isterminated when the rate of change drops to zero, indicating that thesample is dry.

The invention also lies in apparatus for evaporating a sample comprisedof solid material dissolved or suspended in a liquid, comprising avacuum chamber, a rotor therein, drive means for rotating the rotorrelative to the chamber, a sample holder for containing the sampleconnected to the rotor, transducer means associated with the sampleholder and the rotor for generating a force signal indicative of thecentrifugal force acting on the sample holder as it is rotated at agiven speed, and means for transmitting transducer signals to computingmeans programmed to convert the signal at any instant to a computervalue proportional to weight, the computing means being furtherprogrammed to generate a process control signal for controlling theevaporation process in the chamber.

The force transducer may be a load cell, or a strain gauge, or where thesample holder is movable relative to the rotor, the force transducer maybe a position sensor adapted to produce a signal indicating the positionof the sample holder relative to the rotor, as determined by theinstantaneous centrifugal force acting on the sample holder, causing itto move relative to the rotor.

Where the movement is permitted, preferably resilient means is providedwhich resists the movement of the sample holder relative to the rotor.

A plurality of sample holders may be mounted on the rotor and a forcetransducer is provided for selected ones, or all of, the holders.

The weight of the sample can be calculated from a force value by takingaccount of the centrifugal force and deducting the known weight of theholder, but an equally useful measurement is that of the rate of changeof weight. This is a direct measurement of mass flow rate and can beused to monitor the progress of the evaporation and to reduce the heatwhen the rate starts to decline, when the samples are nearly dry and toshut the system down when it drops to 0 indicating that the samples aredry.

According to another aspect of the invention in the processing ofsamples in a centrifugal evaporator in which the samples are dissolvedor suspended in liquids of differing volatility, any imbalance causedduring spinning of the rotor and resulting in unwanted vibration is atleast partially compensated for by associating with the rotor anautomatic balancing unit.

The invention therefore also lies in comprising a vacuum chamber, arotor mounted therein for rotation in use about a generally verticalaxis, a drive means for rotating the rotor, at least two sample holdersmounted on the rotor, each sample holder being in use about a generallyhorizontal axis in a radial manner relative to the axis of rotation, abearing raceway incorporating a plurality of ball bearings which do notfully occupy the circumferential extent of the raceway and which inrotation are automatically distributed around the raceway to counteractany imbalance forces experienced by the raceway, the bearing racewaybeing mounted to the rotor or a spindle driving the rotor, thereby toreduce any imbalance caused during the spinning of the rotor as resultof differential evaporation of liquids from the sample holder.

The ball bearings may be formed from a high density material such asTungsten or depleted Uranium.

The invention also lies in a method of measuring the weight of a liquidsample in a sample holder attached to a rotor in a vacuum chamber of anevaporating centrifuge, comprising the steps of mounting a transducer tomonitor the force acting on the sample holder relative to the rotorduring rotation, supplying a force signal to a computing means havingstored therein a stored weight value corresponding to the empty weightof the sample holder, the computing means being programmed to convertthe force signal to a weight value for a given speed of rotation of therotor, the computing means being further programmed to deduct from thecomputed weight value said stored weight value.

The method may further comprise the steps of monitoring the speed ofrotation of the rotor, and supplying a speed signal to the computingmeans for computing said weight signal.

The weight measuring method may be enhanced by mounting to the rotatingparts of the apparatus an automatic balancing aid, to counteract any outof balance force arising from differential evaporation of samples.

Only limited space is available within apparatus as described herein forlaboratory use and the like, and therefore it is to advantage to userolling elements constructed from dense materials such as Tungsten ordepleted Uranium, since this allows the overall size of the raceway tobe reduced both in depth and diameter, due to the increased mass of therolling elements obtained by using high density materials therefore.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described, by way of example only, withreference to the accompanying drawings in which:

FIG. 1 is a schematic side view of a centrifugal evaporation systemincorporating a force measuring transducer in accordance with theinvention;

FIG. 2 is a perspective view of a dissembled automatic balancing unitassociated with the rotor of FIG. 1, and

FIGS. 3 and 4 are block schematic diagrams showing the probes andcontrol system as employed in an evaporator such as shown in FIG. 1embodying the invention.

DETAILED DESCRIPTION

FIG. 1 illustrates a centrifugal evaporator embodying the inventiondescribed and claimed herein.

The samples in FIG. 1 are contained in plates or blocks 4 in which thereare numerous sample wells (not shown), commonly referred to as deep-wellmicrotitre plates or blocks.

When the sample holder rotor 5A and shaft 5B rotates, driven by a motor5C, which may be inside but more usually external to the chamber (14),the sample blocks swing out to a position in which the sample wells arehorizontal, under the influence of centrifugal force.

The sample blocks are pivoted about swivel pins 13 and the blocks areheld with the wells vertical for loading in the a stationary evaporator.Vacuum is then applied to the evaporator chamber 14 via pipe 9 from avapour condenser 26 which in turn is pumped via pipe 10 by a vacuum pump28.

Heat is applied to the rotating sample blocks 4 by a heater 1 in theform of a high temperature infra-red radiation source, and a beam ofradiant heat energy 2 passes through a window 3 of heat-transparentmaterial such as quartz which is sealed into the wall of the vacuumchamber 14 and reaches the sample holder as illustrated.

A temperature sensor or probe 15 is placed in one of the sample wells,or otherwise placed in close proximity to the wells in one of the sampleblocks, and is connected to a transmitter 11 which transmits signalscorresponding to the sample temperature to an aerial and feedthrough 6inside and extending through the chamber wall, and which is connected toan external receiver and decoder 16. The decoder includes dataprocessing and computing facilities, as required, and indicates thesample temperature by a display (not shown) and, if required, can beprogrammed to generate electrical signals to control the operation ofthe heater in order to increase or decrease the heat energy to keep thesamples at desired temperatures during the process. Such control signalsare supplied to the heater 1 via a connection 17.

It is important that as far as possible all the samples evaporate at thesame rate. To achieve this, all the samples should receive the same heatinput by directing the heat to them so as to heat all the sample holdersuniformly. A common form of sample holder is the deep-well microtitreplate or block 4, in which there are typically 96 wells.

Each block 4 is mounted on the swivel pin 13 so that when it isinitially loaded onto a stationary rotor 5A the open ends of the wellsface upwards; but as soon as the rotor 5A is rotated at a sufficientspeed, the blocks 4 swing into a position in which the wells are almosthorizontal, as is in fact shown in FIG. 1. In this position theinfra-red beam 2 is directed horizontally onto the closed ends of thesample wells, in which configuration it is possible to achieve uniformheating of the wells.

Even with perfectly uniform heat input the samples will not evaporate ata uniform rate because of a so-called “cold neighbour effect”. If thesamples are in thermal contact with each other, as is the case forexample in a microtitre plate or block 4, the outer samples only haveevaporating (and therefore “cold”) neighbours on three or (for cornersamples) two sides, and therefore do not lose as much heat to theirneighbours as those in the centre which have four “cold” neighbours.Also two of an outside sample's neighbours will generally be less coldthan those of the inner samples. Outer samples therefore can evaporatefaster than centrally located samples.

This effect can be reduced or eliminated by reducing the heat input tothe outer samples. A simple way of doing this in the preferred infra-redheating case, is to provide graduated shading from the infra-red beam 2by, for example, placing a metal screen between the sample holder andthe heater 1. The screen contains graduated perforations so that thosein the outer region transmit much less radiation than do those in thecentral region, and those in intermediate regions, which have anintermediate size thereby transmit greater quantities of heat than dothe outer ones.

Although the sample holder (4) illustrated is described as being adeep-well microtitre block or plate, the same techniques may be employedto obtain uniform temperature and graduated heating as described above,when using arrays of tubes, bottles or vials in holders which swing outon swivels in a similar manner.

The power of the heater 1 is controlled by measuring sample temperatureor chamber pressure and taking appropriate steps to raise or lower theheater power. Thus at the start of the process a high heat input isrequired, but as the samples approach dryness the evaporation rate willreduce and the sample temperature will start to rise so that the heatinput must be reduced to avoid overheating the sample, and when thesamples are dry, the heating must be discontinued.

The vapour condenser 26 is used in centrifugal evaporation equipment toincrease pumping speed for the liquid being evaporated and to protectthe vacuum pump 28 from vapours which might impair its efficiency. Sucha condenser is a vessel held at low temperatures at which the vapoursbeing evaporated condense or solidify.

If the condenser 26 is located between the vacuum pump 28 and theevaporation chamber 14, as shown in FIG. 1, the pressure in the chamber14 cannot be reduced below the vapour pressure of any condensed liquidremaining in the condenser 26. This is due to the evaporation ofcondensed material which will take place in the condenser if the systempressure is reduced to a level approaching the vapour pressure of thecondensed material left in the condenser 26. This phenomenon, especiallyif a more volatile material has been left in the condenser 26 from aprevious run, can make chamber pressure a rather insensitive techniquefor sensing sample temperature at the end of evaporation to indicatewhen the samples are dry, and it may be unreliable as a means fordetermining when the equipment can be shut down.

The measurement of vapour flow rate is a more useful monitor of theevaporation process. By thus monitoring flow rate, information can beobtained about a process to indicate when to turn off the heater, sincewhen the samples are nearly dry the flow rate will become low. Thisenables equipment to be reliably shut down when the process is finished(ie the samples are dry).

Flow rate through the condenser or the pipe 9 between the chamber 14 andthe condenser 26 can be monitored by any convenient technique.

In accordance with the present invention, a load cell 19 is attachedbetween each plate or block 4 and its support. The load cell produces anelectrical signal indicative of the horizontal force on the block which,when the rotor is spinning, will be proportional to the combined weightof the sample and the sample holding assembly. Since the latter isconstant the sample weight can readily be obtained. Of course, theapparent weight will be exaggerated by a factor due to the centrifugalforce, but this factor will not vary for a given rotor speed. In somearrangements the rotor speed may be kept constant; however, where thespeed is variable it is important also to monitor the rotational speedof the rotor and sample holders.

FIG. 3 shows the important components of the monitoring system for achamber 14, such as shown in FIG. 1. Each temperature probe 15 connectsto an input of a signal processor 50, the output of which is digitisedby an A/D converter 52 for supply to a microprocessor 54 which handlesthe modulation of a radio signal in a transmitter 56 to which signalsare supplied from the microprocessor for radiation by an antenna 58.Power for the system may be from a battery or a mains supply 60. Exceptfor the probe 15 and the antenna 58, all the units shown in FIG. 3 maybe housed within a housing located on the sample holder rotor 5A, sothat there is no relative movement between the housing and the probe 15.The chamber 14 must be constructed so that at least part of its wall iscapable of transmitting the radio signals from the antenna.

The force signals from the load cell 19 are processed and transmitted toa receiver and decoder outside the chamber via a separate transmissionchannel on the signal processing circuit of FIG. 3.

A receiver and control system for locating outside the chamber 14 isshown in FIG. 4.

Here the receiver antenna 62 feeds radio signals to a receiver anddecoder 64 which supplies decoded digital data signals (corresponding tothose from the A/D converter 52 in FIG. 3), to a second microprocessor66. This controls the supply of digital signals to a motor controller 68which controls the speed of rotation of the drive motor 5C (also shownin FIG. 1). A tacho-generator 70 is attached to the motor shaft 72 andprovides a speed signal for the microprocessor 66.

An infra-red heater 1 (see also FIG. 1) is controlled by a powercontroller 74 which in turn is controlled by signals from themicroprocessor 66, to reduce the heat output from the heater 1 as anevaporation process progresses, so as to reduce the risk of overheatingas samples dry and are no longer cooled by evaporative cooling effects.

The vacuum pump 28 of FIG. 1 is shown associated to the chamber 14 via apipeline 76 which includes a valve 78 also under the control of signalsfrom the microprocessor 66. The latter includes a memory in whichoperating system software and data relating to different volatileliquids are stored and a data entry keyboard or other device 80 allowsdata to be entered initially and volatile components to be identified tothe system. A display screen 82 assists in the entry of data and thedisplay of monitored values of temperature from probe 15 and pressurefrom a probe 84 in the chamber, and of force (and therefore bycomputation weight) from load cell 19.

The memory also stores the values of force signals from load cell 19when an empty standard sample holder is rotating around the chamber, andfactors by which force signal values can be converted to weight fordifferent rotational speeds (from the tacho 70) and therefore differentg-forces. It can also store weight values for empty sample holders suchas mitrotitre plates or blocks.

Power for the system of FIG. 4 may be from a battery or a mains drivenpower supply 86.

Experiments have shown that weights of samples in holders weighing up to1200 gm can be determined using this apparatus and approved to anaccuracy of better than 1 gm.

The microprocessor 66 can be programmed to compute the rate of change ofweight with time, and this or the monitored force value can be used todetermine when the samples have been fully evaporated, and therefore thepoint at which the samples are completely dry. This enables the correctmoment to be identified when to switch off heat to the samples.

FIG. 2 shows a proprietary automatic balancing unit 20,22 which isfitted to the rotor shaft 5B as close as possible to the rotor 5Acarrying the plates or blocks 4. Vibration caused by rotor imbalance islikely to occur when solvents of different volatility are used for thesamples.

The unit 20, 22 may be an Auto-Balancing unit produced by the bearingmanufacturing company SKF.

As shown in FIG. 2, the unit comprises inner and outer raceways 20 and22 between which a number of loose ball bearings 24 are freely movable.The ball bearings distribute themselves automatically to counteract theimbalance in the rotor shaft 5.

In the known forms of autobalancing units of this type the ball bearings24 are normally made of steel, but a greater balancing capability can beobtained by using balls of a heavier material, for example Tungsten ordepleted Uranium. The use of higher density metal for the balls allowsthe same out-of-balance forces to be counteracted using a racewayassembly 20, 22 of small dimensions, both in width and diameter.

One unit which has been used to advantage is the Auto-Balancing deviceproduced by the company SKF such as is described in WO98/01733.

What is claimed is:
 1. A method of evaporating a liquid sample containedin a sample holder which is mounted within a chamber and rotated by arotor at speeds which are monitored to produce speed signals thereinduring the evaporation so that a centrifugal force is exerted on thecontents of the sample holder during the process whilst a pressure belowatmospheric is maintained in the chamber in manner known per se, so asto leave as a residue any solid material dissolved or otherwise mixed inthe liquid forming the sample, comprising the steps of: mounting atransducer to monitor the centrifugal force acting on the sample holderrelative to the rotor when rotating at a given speed and obtaining aforce signal therefrom, supplying the force signal to a computing means,programming the computing means to compute a value equivalent to thecentrifugal force exerted on the sample holder due to rotation of therotor at said given speed, further programming the computing means tocomputer a weight of the liquid sample and sample holder from the forcesignal using the computer centrifugal force, and further programming thecomputing means to generate a control signal for controlling theevaporation process in dependence on the computed weight, wherein thecomputing means includes a microprocessor adapted to rotate with therotor.
 2. A method as claimed in claim 1, further comprising the stepsof mounting a second transducer to monitor the speed of rotation of therotor, obtaining a speed signal therefrom, and supplying the speedsignal to the computing means for computing said weight.
 3. A method ofmeasuring the weight of a liquid sample in a sample holder attached to arotor in a vacuum chamber of a centrifugal evaporator, comprising thesteps of mounting a force transducer to monitor the force acting on thesample holder relative to the rotor during rotation, supplying a forcesignal from the transducer to a computing means having stored therein astored weight value corresponding to the empty weight of the sampleholder, the computing means being programmed to convert the force signalto a computed weight of the liquid sample and sample holder for a givenspeed of rotation of the rotor, the computing means being furtherprogrammed to deduct said stored weight value from the computed weight,the computing means comprising a microprocessor adapted to rotate withthe rotor.
 4. A method as claimed in claim 1, wherein the computingmeans is programmed to convert the force signal from the transducer intoa form suitable for transmission to an external receiver.
 5. A method asclaimed in claim 4, wherein the computing means converts the forcesignal from the transducer into a digital signal by which a carriersignal is modulated to effect the said transmission.
 6. A method asclaimed in claim 1, wherein the force and speed signals are producedcontinuously and the weight and centrifugal force are continuouslycomputed therefrom.
 7. A method as claimed in claim 6, wherein thecomputing means has stored therein a value equivalent to the knownweight of the sample holder, and is further programmed to compute avalue equivalent to the weight of the contents of the holder bydeducting from the computed weight a value equivalent to the knownweight of the sample holder.
 8. A method as claimed in claim 1, whereinthe computing means computes the rate of change of the computed weight.9. A method as claimed in claim 1, further comprising a step of heatingthe sample during rotation in the chamber to increase the rate ofevaporation.
 10. A method as claimed in claim 9, comprising a step ofcontrolling a supply of heat to the sample in dependence on the computedweight.
 11. A method as claimed in claim 8, comprising a step ofcontrolling a supply of heat in dependence on the computed rate ofchange of weight.
 12. A method as claimed in claim 11, wherein thesupply of heat is reduced as the rate of change of weight with timestarts to decline, and the evaporation process is terminated when therate of change drops to zero, indicating that the sample is dry. 13.Apparatus for evaporating a sample comprised of solid material dissolvedor suspended in a liquid, comprising a vacuum chamber, a rotor therein,drive means for rotating the rotor relative to the chamber, a sampleholder for containing the sample and connected to the rotor, forcetransducer means associated with the sample holder and the rotor forgenerating a force signal indicative of the centrifugal force acting onthe sample holder when rotated at a given speed, and means for supplyingthe force signal to computing means external of the rotor programmed toconvert the force signal at any instant to a computed value proportionalto weight of the sample and sample holder, the computing means beingfurther programmed to generate a process control signal for controllingthe evaporation process in the chamber and including a microprocessorrotatable with the rotor.
 14. Apparatus as claimed in claim 13, furthercomprising second transducer means associated with the rotor forgenerating a speed signal corresponding to the speed of rotation of therotor, the speed signal being transmitted to the computing means forcomputing said weight.
 15. Apparatus as claimed in claim 13, wherein theforce transducer means is a load cell.
 16. Apparatus as claimed in claim13, wherein the force transducer means is a strain gauge.
 17. Apparatusas claimed in claim 13, wherein the sample holder is movable relative tothe rotor, and further comprising a position sensor adapted to produce asignal indicating the position of the sample holder relative to therotor, as determined by the centrifugal force acting on the sampleholder, causing the sample holder to move relative to the rotor. 18.Apparatus as claimed in claim 17 wherein a resilient means resists themovement of the sample holder relative to the rotor.
 19. Apparatus asclaimed in claim 13, wherein a plurality of sample holders are mountedon the rotor and a force transducer is provided for at least eachselected ones of the holders.
 20. Apparatus as claimed in claim 13,wherein a mechanical device is attached to the rotor, or to a spindle onwhich the rotor is carried and by which the rotor is rotated, whichdevice automatically adjusts the device centre of mass in response toout-of-balance forces acting on the rotor due to differentialevaporation of samples.
 21. Apparatus according to claim 13 in whichthere are at least two sample holders mounted on the rotor, each sampleholder being pivotal in use about a generally horizontal axis in aradial direction relative to the axis of rotation, and furthercomprising a bearing raceway incorporating a plurality of ball bearingswhich do not fully occupy the raceway and which ball bearings inrotation are automatically distributed around the raceway to counteractany imbalance forces, the raceway being mounted to the rotor or aspindle driving the rotor, thereby to reduce any imbalance caused duringrotation of the rotor as result of differential evaporation of liquidsfrom each sample holder.
 22. Apparatus as claimed in claim 21, whereinthe ball bearings are formed from a high density material of a groupcomprising Tungsten and depleted Uranium.
 23. A method as claimed inclaim 3, further comprising the steps of monitoring the speed ofrotation of the rotor, and supplying a speed signal to the computingmeans for computing said computed weight.